monitor legacy postgresql backup system in prometheus

in #40950, i have tried and failed to deploy barman as a replacement for our legacy database backup system, in the hope of reusing its exporter and not having to port our stuff to it.

now we need to figure out another way, in the short term.

we need to somehow replace or port dsa-check-backuppg to prometheus.

keep in mind that thing as side effects! from what i remember, it seems it rotates files as well, so maybe we want to split it up into something that rotates file and another that exports metrics, to keep our sanity.